1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
A lens hood also acts as an umbrella for your lens. To some extent it can keep rain drops or dust from landing on the front element. This is especially true for longer lenses, like an 80-200 zoom, that have deeper lens hoods. So, start looking for situations where lens flare can hurt you.